This invention relates to a two stage master cylinder having a two diameter piston or two concentric pistons whose initial movement supplies a brake system with a large volume of hydraulic fluid at a relatively low pressure for bringing the brake shoes or pads into contact with brake drum or rotors and upon subsequent movement with a relatively small volume of hydraulic fluid at a high pressure for actually effecting a brake application.
An early example of a two stage master cylinder is found in U.S. Pat. No. 1,892,335 wherein lost motion associated with mechanical linkages is essentially overcome through the use of a two step braking process. In this braking operation a high volume of hydraulic fluid moves the brake shoes into contact with the brake drums and thereafter permits a smaller amount of fluid at a high pressure to effect a brake application.
In an effort to improve the transitional operation of two stage servomotors, various valving arrangements such as disclosed in U.S. Pat. Nos. 3,631,676 (Krusemark) and 3,802,199 (Hagberg Jr.), allow an operational input force to be applied to a smaller diameter piston area when a transition pressure is created in the system. Unfortunately, the transition pressure still acts on a larger diameter during the remaining brake pressure range.
U.S. Pat. No. 3,667,229 (Cresto), illustrates a valve which eliminates the input force acting on the large diameter above the transition pressure. Unfortunately, this transition pressure occurs during high pressure fluid production. When the transition occurs, the brake pedal moves through an increased rate of angular travel giving a driver the impression of brake fade or failure.